Recently, there has been an increasing interest in energy storage technology. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for the research and development of electrochemical devices is increasing.
In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause electrical short between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, as shown in FIG. 1, a separator 10 having a porous coating layer formed by coating a mixture of inorganic particles 3 and a binder polymer 5 on at least one surface of a porous substrate 1 has been proposed. In the separator, the inorganic particles 3 present in the porous coating layer formed on the porous substrate 1 act as a spacer capable of maintaining the physical form of the porous coating layer, thereby preventing the porous substrate from thermal shrinkage even if electrochemical devices overheat. Also, an interstitial volume is present between the inorganic particles to form a fine pore.
Such a separator is required to be adhesive to an electrode in a stacking and folding process, and therefore, it is preferred that an adhesive layer is significantly exposed on the porous substrate layer of the separator for good adhesion with the electrode. In this regard, there is a need to effectively form an adhesive layer on the porous substrate layer of a separator.